Mosquitoes are important vectors of several tropical diseases, including malaria, filariasis, and a series of viral diseases such as dengue, Japanese encephalitis, West Nile virus, and yellow fever. Of these, malaria-transmitting species are highly important. Globally, an estimated 200-300 million people are affected by malaria, of which 1.5-2.7 million die each year. Over 90% of these cases occur in sub-Saharan Africa and children under-five years are worst hit by the disease. The burden malaria imposes on the economy is substantial. The high morbidity and mortality in some countries with intense transmission has reduced annual per capita income by up to 1.3%. In fact, a 10% reduction in malaria has been associated with 0.3% higher growth in economy. Many man-hours are lost each day from those suffering from malaria or looking after such patients. Pregnant women infected with malaria suffer severe anaemia with up to 800,000 infantile mortalities, a substantial number of miscarriages, and very low birth weight babies, annually. In addition, the spread of drug resistant malaria strains substantially raises the cost of treatment.
Malaria is transmitted by Anopheles mosquitoes. Of about 380 species around the world, some 60 species are sufficiently attracted to humans to act as vectors of malaria. Most cases of the disease in sub-Saharan Africa are transmitted by An. gambiae Giles complex and An. funestus Giles that are strongly attracted to human hosts (i.e. are highly anthropophilic). Control of the disease is through chemotherapy, chemoprophylaxis and vector control. Vector control includes environment management, biological control, use of insecticides for adult and larval control, and personal protection (e.g. using repellents and mosquito nets). Chemical-based methods of vector control focus largely on repellant compounds and compositions. Unfortunately, the use of chemical repellants frequently requires widespread application of chemicals. In addition, many repellants are associated with adverse medical side effects. For example, N.N-diethyl-meta-toluamide (DEET) is a widely used mosquito repellant that, unfortunately, has been identified as an irritant and is known to cause allergic reactions, seizures, and other conditions.
unfortunately, has been identified as an irritant and is known to cause allergic reactions, seizures, and other conditions.
The use of attractants in vector control has received much less attention compared with the use of repellants, in part because of a lack of candidates that are sufficiently attractive to the targeted insects. Carbon dioxide present in the expired breath of mosquito hosts is the best known olfactory stimulant of mosquitoes. Other known host odour constituents include 1-octen-3-ol, lactic acid, 4-methylphenol, and fatty acids associated with activity of microflora present on host skin. However, none of these with or without CO2 account for the strong attraction of Afrotropical malaria vectors to their preferred hosts. A review of the effects of various host odours can be found, for example, in Takken et al., “Odor-Mediated Behavior of Afrotropical Malaria Mosquitos,” Annu. Rev. Entomol. (1999) 44:131-157.
An ideal method for controlling mosquito and other blood-feeding insect populations would incorporate insect attractive compositions. Such insect attractant compositions would ideally contain readily available compounds that are non-toxic at insect-attractant concentrations.